1. Field of the Invention
The present invention relates to a hydrocarbon sensor and a method for producing the same.
2. Description of the Related Art
Some known hydrocarbon sensors are capable of detecting hydrocarbon in a living environment and hydrocarbon in exhaust gas from an automobile engine, a heater, and catalytic combustion equipment, and can be used for combustion control (leanburn) of a combustion engine or combustion equipment.
As a sensor for measuring or detecting hydrocarbon, a hydrocarbon sensor is known, which includes a thin substrate made of a proton conductor that is a solid electrolyte and two electrode layers made of platinum disposed on both sides of the substrate so as to be opposed to each other. In such a hydrocarbon sensor, hydrocarbon in an atmosphere to be measured is dissociated at an anode to generate protons. The protons move in the substrate made of an electrolyte. The sensor detects the protons as a voltage or a current flowing between the electrodes.
In order to use the above-mentioned hydrocarbon sensor for a combustion engine and combustion equipment, it is required to provide a proton conductor made of an oxide that can be used at room temperature or higher. In recent years, as a proton conductor made of an oxide, CaZr0.9In0.1O3xe2x88x92xcex1 (where xcex1 indicates a stoichiometric deficiency of oxygen, which applies to the rest of the specification) that is a calcium zirconium type oxide, has been developed, and an attempt has been made to apply such a proton conductor to a hydrocarbon sensor.
The above-mentioned calcium zirconium type oxide has a small proton conductivity (e.g., about 5xc3x9710xe2x88x924 S/cm at 600xc2x0 C.). Therefore, the inventors of the present invention have proposed a hydrocarbon sensor of a limiting current type (constant-potential electrolysis type) using a barium cerium type oxide that exhibits a high proton conductivity (see JP 10(1998)-300718 A). The sensor responds to hydrocarbon satisfactorily, and substantially linearly can detect hydrocarbon on the order of several ppm to several % in the absence of oxygen.
However, in the case of hydrocarbon of low concentration (e.g., 10 ppm or less), the output of the sensor using a barium cerium type oxide is influenced by the concentration of oxygen. This is because a barium cerium type oxide has a property of conducting oxide ions, and the output of the sensor fluctuates due to the oxygen passed through a cathode. Therefore, the inventors of the present invention have developed a sensor that inhibits the entry of oxygen by using a cathode mainly containing metallic aluminum (see JP 11(1999)-337518 A). The effect of a cathode containing metallic aluminum is so great that the output of the sensor using such a cathode does not increase even when a small amount of oxygen is mixed in the gas to be measured.
The above-mentioned sensor has a problem in that a hydrocarbon detection ability is decreased in the case of oxygen of high concentration. In the case where exhaust gas from an automobile engine is purified with a catalyst, when the performance of the catalyst is degraded, a hydrocarbon (HC) component of high concentration and oxygen of high concentration may be mixed in the exhaust gas. Thus, a conventional sensor does not have characteristics sufficient for detecting the degradation of a catalyst for purifying exhaust gas. Furthermore, in the case of measuring hydrocarbon in exhaust gas, there is a demand for a sensor enduring a heat cycle due to remarkable changes in temperature. In the case of a conventional sensor using an Al-containing electrode, Al forms a nonconductor during a heat cycle to degrade characteristics.
Therefore, with the foregoing in mind, it is a first object of the present invention to provide a hydrocarbon sensor that is unlikely to be influenced by oxygen, in which characteristics are unlikely to be degraded by heat.
It is a second object of the present invention to provide a method for producing a hydrocarbon sensor that is unlikely to be influenced by oxygen.
In a conventional hydrocarbon sensor, Pt or Au is used alone as a conductive adhesive between an electrode and an output lead. However, in such a conventional hydrocarbon sensor, peeling of the electrode and disconnection of the lead are likely to occur. In order to solve this problem, it is a third object of the present invention to provide a highly reliable hydrocarbon sensor.
In order to achieve the above-mentioned object, a first hydrocarbon sensor of the present invention includes a substrate made of a solid electrolyte that conducts protons, and a pair of electrodes formed on the substrate, wherein at least one electrode of the pair electrodes contains Au and Al, and assuming that a content of an Al simple substance (Al metal) in the at least one electrode is xe2x80x9caxe2x80x9d mol %, and a content of aluminum oxide (Al2O3) in the at least one electrode is xe2x80x9cbxe2x80x9d mol %, xe2x80x9caxe2x80x9d and xe2x80x9cbxe2x80x9d satisfy a relationship: a+2bxe2x89xa67. Because of this configuration, the first hydrocarbon sensor is unlikely to be influenced by oxygen, and its characteristics are unlikely to degrade due to heat.
In the first hydrocarbon sensor, the at least one electrode may contain at least one metal selected from the group consisting of an AuAl2 alloy and an Au simple substance (Au metal) in a ratio of at least 50 mol %. More specifically, the at least one electrode may contain both an AuAl2 alloy and an Au simple substance in a total molar ratio of at least 50 mol %. The metal with these compositions has a high melting point, which allows an electrode with high heat resistance to be obtained.
In the first hydrocarbon sensor, the at least one electrode may contain AuAl2 and an Au simple substance in a molar ratio of AuAl2:Au=X:1xe2x88x92X, where 0.6xe2x89xa6Xxe2x89xa61. According to this configuration, a hydrocarbon sensor provided with an electrode having a particularly high oxygen blocking ability can be obtained.
Furthermore, a second hydrocarbon sensor of the present invention includes a substrate made of a solid electrolyte that conducts protons, a pair of electrodes formed on the substrate, and leads connected to the electrodes, wherein at least one electrode of the pair of electrodes contains Au and Al, and the at least one electrode and the lead are connected to each other via a conductive adhesive containing Pt and Au or a conductive adhesive containing Al and Au. In the second hydrocarbon sensor, peeling of the electrode and disconnection of the lead are unlikely to occur, and high reliability is ensured.
In the second hydrocarbon sensor, the at least one electrode and the lead may be connected to each other via a conductive adhesive containing Al and Au, and a component of the at least one electrode may be the same as a component of metal contained in the conductive adhesive. More specifically, a metal element contained in the at least one electrode may be the same as that contained in a conductive adhesive. According to this configuration, the electrode is integrated with the conductive adhesive, whereby a higher adhesion strength can be obtained.
Furthermore, a first method for producing a hydrocarbon sensor including a substrate made of a solid electrolyte that conducts protons, and an electrode formed on the substrate, includes coating the substrate with a paste containing Au particles and Al particles, followed by baking, thereby forming the electrode containing Au and Al. According to the first production method, a hydrocarbon sensor easily can be produced that is unlikely to be influenced by oxygen.
In the first production method, a content of an Al simple substance in the electrode immediately after baking may be 7 mol % or less. According to this configuration, a hydrocarbon sensor can be produced in which characteristics are unlikely to degrade due to heat.
In the first production method, the baking may be conducted in an oxygen-free atmosphere. According to this configuration, a hydrocarbon sensor with a particularly high oxygen blocking ability can be produced.
In the first production method, the oxygen-free atmosphere may be composed of at least one gas selected from the group consisting of nitrogen gas, argon gas, helium gas, and hydrogen gas.
Furthermore, a second method for producing a hydrocarbon sensor including a substrate made of a solid electrolyte that conducts protons, an electrode formed on the substrate, and a lead connected to the electrode, includes connecting the electrode to the lead via a conductive adhesive, followed by baking in an oxygen-free atmosphere, wherein the electrode contains Au and Al. According to the second production method, a hydrocarbon sensor easily can be produced, which is unlikely to be influenced by oxygen, and in which characteristics are unlikely to degrade due to heat and an adhesion strength between the lead and the electrode is high.
In the second production method, the conductive adhesive may contain Pt and Au or contain Al and Au. According to this configuration, a hydrocarbon sensor with a higher adhesion strength between the lead and the electrode can be produced.
In the second production method, the oxygen-free atmosphere in the lead connection process may be composed of at least one gas selected from the group consisting of nitrogen gas, argon gas, helium gas, and hydrogen gas.
The hydrocarbon sensor according to the present invention can be used for detecting hydrocarbon and measuring the concentration thereof in a temperature range of 300xc2x0 C. to a high temperature (e.g., 800xc2x0 C.). More specifically, the hydrocarbon sensor is capable of detecting hydrocarbon in a living environment, and hydrocarbon in exhaust gas from an automobile engine, a heater, and catalytic combustion equipment, and can be used for combustion control (leanburn) of a combustion engine and combustion equipment.
These and other advantages of the present invention will become apparent to those skilled in the art upon reading and understanding the following detailed description with reference to the accompanying figures.